"We know that cigarettes from around the world vary in their ingredients and the way they are produced," says Dr Jim Pirkle of the US Centers for Disease Control and Prevention, who heads a lab using a mass spectrometer to measure levels of chemicals in people's bodies.

"All of these cigarettes contain harmful levels of carcinogens, but these findings show that amounts of tobacco-specific nitrosamines differ from country to country, and US brands are the highest in the study," says Pirkle.

CDC's David Ashley and colleagues did in-depth tests using 126 smokers in the United States, Canada, Britain and Australia.

"Seventeen eligible cigarette brands (between three and five brands from each country) were selected on the basis of national sales and nicotine yield to identify popular brands with a range of ventilation," the researchers write. Ventilation is how much air is mixed in with the smoke from the cigarette as it is inhaled.

The volunteers had their saliva and urine tested and also turned over their used cigarette butts to the researchers.

Blend variation

These were all tested for nicotine and for the chemicals 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK for short) and the breakdown product of NNK in the body, called NNAL.

These cancer-causing agents are known as tobacco-specific nitrosamines or TSNAs.

"We have shown a direct association between the 24-hour mouth-level exposure of NNK resulting from cigarette smoking and the concentration of its primary metabolite, NNAL, in the urine of smokers," the researchers write.

"Internal dose concentrations of urinary NNAL are significantly lower in smokers in countries that have lower TSNA levels in cigarettes such as Canada and Australia in contrast to countries that have high levels of these carcinogens in cigarettes, such as the United States."

The popular US cigarette brands studied contained 'American blend' tobacco, known to contain higher TSNA levels than the "bright" tobacco used in the most popular Australian, Canadian, and British brands.

Australian and Canadian smokers got more nicotine than US and British smokers, but not of TSNAs.

The World Health Organization says 5 million people die each year from tobacco-related heart attacks, strokes and cancers. Another 430,000 adults die annually from breathing second-hand smoke.